As the demand for integrated circuits having ever-smaller device features continues to increase, the need for high power illumination sources used for inspection of these ever-shrinking devices continues to grow. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma light sources are capable of producing high-power broadband light Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting light. This effect is typically referred to as “pumping” the plasma.
Generally, the most commonly used approach to increasing collectible output power of laser-sustained plasma source is to increase the laser pump power. There exist a variety of additional techniques for increasing the power, which may be used in conjunction with increased pump power. The additional techniques include optimizing laser pump wavelength, optimizing pumping NA, using different lamp gas fill mixtures, aberration control of the light source and optimizing lamp temperature.
Increasing the laser pump power of the laser-sustained source has its limitations. The collectible power of the given laser-sustained source only increases linearly with laser pump power for low pump power (e.g., power is less than 3-4 kW). In contrast, for higher pump power (e.g., power greater than 3-4 kW) saturation is observed in the collective power. This effect is observed in FIG. 1A, which depicts a graph 10 of collectible power versus laser pump power. As observed in FIG. 1A, collectible power begins to saturate at pump powers at approximately 3 kW. The saturation at higher pump power occurs in laser-sustained sources because the associated plasma begins to grow in size with brightness saturation occurring at the center of the plasma. The saturation of collectible power with increasing laser pump power leads to very large laser powers used in a given laser-sustained plasma source. For example, in order to achieve a 140 W collectible power output a laser pump power of approximately 30 kW may be necessary. Using such high laser pump powers, a laser-sustained plasma source is challenging to implement as a result of optical damage and the thermal management of the source. As a result, it would be desirable to provide a system and method for curing defects such as those of the identified above.